For the past several years, extensive research has been devoted to the development of intermetallic materials, such as titanium aluminides, for use in the manufacture of light weight structural components capable of withstanding high temperatures/stresses. Such components are represented, for example, by blades, vanes, disks, shafts, casings, and other components of the turbine section of modern gas turbine engines where higher gas and resultant component temperatures are desired to increase engine thrust/efficiency or other applications requiring lightweight high temperature materials.
Intermetallic materials, such as gamma titanium aluminide, exhibit improved high temperature mechanical properties, including high strength-to-weight ratios, and oxidation resistance relative to conventional high temperature titanium alloys. However, general exploitation of these intermetallic materials has been limited by the lack of strength, room temperature ductility and toughness, as well as the technical challenges associated with processing and fabricating the material into the complex end-use shapes that are exemplified, for example, by the aforementioned turbine components.
The Kampe et al U.S. Pat. No. 4,915,905 issued Apr. 10, 1990 describes in detail the development of various metallurgical processing techniques for improving the low (room) temperature ductility and toughness of intermetallic materials and increasing their high temperature strength. The Kampe et al '905 patent relates to the rapid solidification of metallic matrix composites. In particular, in this patent, an intermetallic-second phase composite is formed; for example, by reacting second phase-forming constituents in the presence of a solvent metal, to form in-situ precipitated second phase particles, such as boride dispersoids, within an intermetallic-containing matrix, such as titanium aluminide. The intermetallic-second phase composite is then subjected to rapid solidification to produce a rapidly solidified composite. Thus, for example, a composite comprising in-situ precipitated TiB.sub.2 particles within a titanium aluminide matrix may be formed and then rapidly solidified to produce a rapidly solidified powder of the composite. The powder is then consolidated by such consolidation techniques as hot isostatic pressing, hot extrusion and superplastic forging to provide near-final (i.e., near-net) shapes.
U.S. Pat. No. 4,836,982 to Brupbacher et al also relates to the rapid solidification of metal matrix composites wherein second phase-forming constituents are reacted in the presence of a solvent metal to form in-situ precipitated second phase particles, such as TiB.sub.2 or TiC, within the solvent metal, such as aluminum.
U.S. Pat. Nos. 4,774,052 and 4,916,029 to Nagle et al are specifically directed toward the production of metal matrix-second phase composites in which the metallic matrix comprises an intermetallic material, such as titanium aluminide. In one embodiment, a first composite is formed which comprises a dispersion of second phase particles, such as TiB.sub.2, within a metal or alloy matrix, such as Al. This composite is then introduced into an additional metal which is reactive with the matrix to form an intermetallic matrix. For example, a first composite comprising a dispersion of TiB.sub.2 particles within an Al matrix may be introduced into molten titanium to form a final composite comprising TiB.sub.2 dispersed within a titanium aluminide matrix. U.S. Pat. No. 4,915,903 to Brupbacher et al describes a modification of the methods taught in the aforementioned Nagle et al patents.
U.S. Pat. Nos. 4,751,048 and 4,916,030 to Christodalou et al relate to the production of metal matrix-second phase composites wherein a first composite which comprises second phase particles dispersed in a metal matrix is diluted in an additional amount of metal to form a final composite of lower second phase loading. For example, a first composite comprising a dispersion of TiB.sub.2 particles within an Al matrix may be introduced into molten titanium to form a final composite comprising TiB.sub.2 dispersed within a titanium aluminide matrix.
U.S. Pat. No. 3,203,794 to Jaffee et al relates to gamma TiAl alloys which are said to maintain hardness and resistance to oxidation at elevated temperatures. The use of alloying additions such as In, Bi, Pb, Sn, Sb, Ag, C, O, Mo, V, Nb, Ta, Zr, Mn, Cr, Fe, W, Co, Ni, Cu, Si, Be, B, Ce, As, S, Te and P is disclosed. However, such additions are said to lower the ductility of the TiAl binary alloys.
An attempt to improve room temperature ductility by alloying intermetallic materials with one or more metals in combination with certain plastic forming techniques is disclosed in the Blackburn U.S. Pat. No. 4,294,615 wherein vanadium was added to a TiAl composition to yield a modified composition of Ti-31 to 36% Al -0 to 4% V (percentages by weight). The modified composition was melted and isothermally forged to shape in a heated die at a slow deformation rate necessitated by the dependency of ductility of the intermetallic material on strain rate. The isothermal forging process is carried out at above 1000.degree. C. such that special die materials (e.g., a Mo alloy known as TZM) must be used. Generally, it is extremely difficult to process TiAl intermetallic materials in this way as a result of their high temperature properties and the dependence of their ductility on strain rate.
A series of U.S. patents comprising U.S. Pat. Nos. 4,836,983; 4,842,817; 4,842,819; 4,842,820; 4,857,268; 4,879,092; 4,897,127; 4,902,474; and 4,916,028, have described attempts to make gamma TiAl intermetallic materials having both a modified stoichiometric ratio of Ti/Al and one or more alloyant additions to improve room temperature strength and ductility. The addition of Cr alone or with Nb, or with Nb and C, is described in the '819; '092 and '028 patents. In making cylindrical shapes from these modified compositions, the alloy was typically first made into an ingot by electro-arc melting. The ingot was melted and melt spun to form rapidly solidified ribbon. The ribbon was placed in a suitable container and hot isostatically pressed (HIP'ped) to form a consolidated cylindrical plug. The plug was placed axially into a central opening of a billet and sealed therein. The billet was heated to 975.degree. C. for 3 hours and extruded through a die to provide a reduction of about 7 to 1. Samples from the extruded plug were removed from the billet and heat treated and aged.
U.S. Pat. No. 4,916,028 (included in the series of patents listed above) also refers to processing the TiAl base alloys as modified to include C, Cr and Nb additions by ingot metallurgy to achieve desirable combinations of ductility, strength and other properties at a lower processing cost than the aforementioned rapid solidification approach. In particular, the ingot metallurgy approach described in the '028 patent involves melting the modified alloy and solidifying it into a hockey puck-shaped ingot of simple geometry and small size (e.g., 2 inches in diameter and 0.5 inch thick), homogenizing the ingot at 1250.degree. C. for 2 hours, enclosing the ingot in a steel annulus, and then hot forging the annulus/ring assembly to provide a 50% reduction in ingot thickness. Tensile specimens cut from the ingot were annealed at various temperatures above 1225.degree. C. prior to tensile testing. Tensile specimens prepared by this ingot metallurgy approach exhibited lower yield strengths but greater ductility than specimens prepared by the rapid solidification approach.
Despite the attempts described hereabove to improve the ductility and strength of intermetallic materials, there is a continuing desire and need in the high performance material-using industries, especially in the gas turbine engine industry, for intermetallic materials which have improved properties or combinations of properties and which are amenable to fabrication into usable, complex engineered end-use shapes on a relatively high volume basis at a relatively low cost. It is an object of the present invention to satisfy these desires and needs.